Description

Manx shearwaters spend most of the year at sea returning to land
only to breed. They nest in burrows and under boulders, and come
ashore only under the hours of darkness in order to evade predators
such as great skuas and great black-backed gulls. They breed
exclusively on islands, usually free of rats Rattus
sp. that depredate eggs, chicks and adults. Manx shearwaters were
believed to have been exterminated from their eponymous colony on
the Calf of Man by the introduction of rats from a wrecked ship in
the late 18th Century. More recently rats and cats
Felis catus were responsible for the extirpation of Manx
shearwaters from Canna (Lochaber). Those few colonies that occur on
islands with rats are generally small and limited in distribution.
The exception is on Rum (Lochaber), where the largest single colony
in the world coexists with rats, though there is evidence
that deleterious impacts are occurring. Coexistence has been
allowed by shearwaters nesting on the slopes of the island's
mountains at altitudes of more than 450m - higher than rats
normally occur, though milder winters in recent years may be
increasing the habitable range of rats into the shearwaters'
range.

Most of the estimated world population of
c.340,000–410,000 pairs of Manx shearwaters breed in
Britain and Ireland. Of the UK population, 40% breed on Rum, and
50% in Pembrokeshire on the adjacent islands of Skomer,
Skokholm and Middleholm (all Dyfed).

Conservation status

Manx shearwater is currently identified as a conservation
priority in the following:

International importance

The UK
population figure (rounded to the nearest hundred) was derived from
data in Mitchell, P.I., Newton, S.F., Ratcliffe, N. and Dunn, T.E.
(eds.) 2004. Seabird Populations of BritainandIreland. Poyser, London. This was
also the source of figures for the Biogeographic and World
populations.

UK population estimates and change 1969-2002 (census data)

Manx shearwaters' nocturnal and subterranean habits have caused
problems for surveyors in the past. Hence, Operation Seafarer and
the SCR Census' estimates of 175,000-300,000 pairs and
250,000-300,000 pairs respectively were based solely on order of
magnitude estimates and should not be compared to results obtained
during Seabird 2000 which represent the first attempt to
survey and quantify accurately the number of Manx shearwaters
breeding in the UK. Surveyors used tape-playback
which involved playing calls of Manx shearwaters to elicit a
response from adults occupying burrows during the day.
Unfortunately, not all adults present at a colony will respond to
the taped calls, thus counts of responses will
underestimate numbers and therefore have to be adjusted
by a response rate measured at the colony. Some colonies were also
surveyed by counting burrow entrances that had visible signs of
use, though this method is difficult or impossible to use in
colonies that are shared with other burrowers, i.e. rabbits and
Atlantic puffins, or where burrow entrances are obscured, i.e.
under boulders or in thick vegetation.

The main gaps in survey coverage in the UK were in the Northern
Isles (where only relict populations remain), Bearasay (Western
Isles), Eigg and Muck (Lochaber) and the Sanda Islands (Argyll and
Bute). However, the combined population of all these islands is
thought to be no more than 1,000 apparently occupied burrows.

Operation Seafarer

(1969-70)

Seabird Colony Register

(1985-88)

Seabird 2000

(1998-2002)

UK Population estimate (AOS)*

N/a

N/a

299,678

% change since previous census

N/a

N/a

N/a

* AOS = Apparently Occupied Sites

For census results for individual countries and Ireland, the
Channel Islands and the Isle of Man see under relevant
sections below.

Distribution/abundance

The Seabird
2000 census provides the most comprehensive recent
assessment of the distribution and abundance of breeding seabirds.
Numbers of Manx shearwater found in different regions, and a
map showing location and size of colonies, are provided in the
Seabird 2000 Manx shearwater
results page (PDF, 1.0 mb).

An
interactive map is available on the NBN Gateway, where you can
filter to display only the Seabird 2000 data. For more
recent, but less comprehensive, coverage view the distribution on
the NBN with all available contributing datasets.

Annual abundance and productivity by geographical
area

With reference to the regional
accounts below please note the following.

Breeding abundance:
graphs of abundance index with 95% Confidence Levels (CLs) are only
shown for a region where the trend produced has been deemed
accurate (see
methods of analysis). Where a trend was thought to be
inaccurate, graphs of abundance at major colonies in a
region may be shown instead, particularly if such colonies
hold greater than 10% of the regional population,
are monitored frequently and may thus help illustrate regional
population fluctuations outwith national censuses.
Occasionally, too few data have been collected regionally to
produce either of these.

Productivity: graphs
of productivity are only shown if analysis of breeding
success data produced a significant result for regional and/or
year effects (again see
methods of analysis). If results were not significant,
then a regional mean productivity value is given.
However, on some occasions too few data are
available from which to provide a meaningful average.
Furthermore, for 11 species where the quality of monitoring data
available was considered high, population viability analysis
was undertaken at the UK level and the results of this are
also reported.

Breeding abundance

The first comprehensive estimates of population size of Manx
shearwater were obtained during the Seabird 2000 Census, when
299,678 pairs were estimated. Over 90% of the UK population is
found on the islands of Rum in Scotland and on Skomer and
Skokholm in Wales. Due to the logistical difficulty in monitoring
this nocturnal and burrow-nesting species, little information
exists from which to derive population trends since Seabird
2000, although the colony on Skomer was
re-surveyed in 2011. However, census methods used in 1998
and 2011 differed markedly, introducing an element of
uncertainty in the results (see 'Wales' tab for
details).

Productivity

Because of the logistical difficulties involved, both in
visiting remote islands and in collecting data from a
nocturnal burrow-nesting seabird, breeding success is
monitored at only a few Manx shearwater colonies in the UK (three
colonies in Wales, two in Scotland and one in Northern Ireland).
Analysis has shown that there is no statistically significant
annual variation in productivity within the sampled colonies, with
Manx shearwaters fledging an average of 0.66 chicks per
breeding pair per year between 1986 and 2015.

Population estimates and change 1969-2002 (census data)

Operation Seafarer

(1969-70)

Seabird Colony Register

(1985-88)

Seabird 2000

(1998-2002)

Population estimate (AOS*)

N/a

N/a

126,545

% change since previous
census

N/a

N/a

N/a

* AOS = Apparently Occupied Sites

Breeding Abundance

The first comprehensive estimates of population size of Manx
shearwater in Scotland were obtained during the Seabird 2000
Census, when 126,545 pairs were estimated. The majority of
these (approximately 120,000 pairs) were found on Rum with a
further 4,803 pairs in the next largest concentration on the island
group of St Kilda (Western Isles). The Treshnish Isles (Argyll
and Bute, 1,283 pairs) was the only other colony holding
over one thousand pairs. Due to the logistical difficulty in
monitoring this nocturnal and burrow-nesting species, no
information exists on the population trends at these large colonies
since Seabird 2000.

Productivity

Productivity data are currently collected mostly
from colonies on Rum with data in some years available
from Canna and Sanday. Analysis showed no statistically
significant variation in average productivity between 1986 and
2015, with approximately 0.69 chicks fledged per AOS per
year. On Canna, one of the SMP key sites, only six potential chicks
fledged from over 750 burrows1. It is not known what
caused this low breeding success. There is some evidence that
predation of nests/chicks by brown rats Rattus norvegicus
on Rum may affect breeding success. For example, in
2007, 25 eggs and 10 chicks showing signs of rat
predation were found in the study area. This problem has been
the subject of ongoing investigation; in most years some signs
of rat activity are recorded although rarely is this
quantified.

Population estimates and change 1969-2002 (census data)

Operation Seafarer

(1969-70)

Seabird Colony Register

(1985-88)

Seabird 2000

(1998-2002)

Population estimate (AOS*)

N/a

N/a

367

% change since previous
census

N/a

N/a

N/a

* AOS = Apparently Occupied Sites

Breeding Abundance

Relatively few Manx shearwaters nest in England with 367
AOS estimated during Seabird 2000. Most of these were
spread around the Isles of Scilly (201 AOS) with Lundy (Devon)
holding the only other colony (166 AOS, though a later
re-assessment of the Seabird 2000 data put this figure at 297
AOS)2. Recently,
eradication of rats from Lundy has benefited Manx
shearwaters; a whole-island survey in 2008 estimated 1,081
AOS2 and further
survey work in 2013 found 3,451 AOS - almost ten times the
English population found during Seabird 2000. A survey of the
Isles of Scilly in 2015 found 439 AOS on the same islands counted
in 2006, however an additional 84 AOS were found at five other
locations3, an increase of 160% from Seabird 2000.

Productivity

No systematic data on the productivity of Manx shearwaters
in England have been submitted to the SMP.

Population estimates and change 1969-2002 (census data)

Operation Seafarer

(1969-70)

Seabird Colony Register

(1985-88)

Seabird 2000

(1998-2002)

Population estimate (AOS*)

N/a

N/a

168,133

% change since previous
census

N/a

N/a

N/a

* AOS = Apparently Occupied Sites

Breeding Abundance

Together with Scotland, Wales holds over 90% of the UK
population of Manx shearwaters. The first comprehensive estimates
of the population size were obtained during the Seabird 2000
Census, when 101,800 pairs were estimated on Skomer, 46,200
pairs on Skokholm and a maximum of 16,183 pairs on Bardsey. Smaller
colonies of 1,000-3,000 pairs were recorded on Middleholm and
Ramsey (Dyfed). Until recently little information existed
on potential population trends. However, a census was carried
out in 2011 on Skomer where the population was estimated to be
316,070 AOS4. This
figure is greatly in excess of the estimate made just 13 years
earlier and would require an increase of approximately 9% per
annum, a very high value for a bird with a low
reproductive rate (c.0.65) and a long period of deferred
maturity. Various reasons for the apparent increase (e.g.
immigration, lowering of age of first breeding, estimation of
response rate) were considered and thought unlikely. However,
survey methods used in 1998 and 2011 differed markedly so it
was concluded that the methods used in one (or both) of the surveys
were sufficiently flawed to account for the difference, or part of
it4.

Productivity

Figure 1:Trend
in breeding productivity (no. of chicks fledged per pair)
of Manx shearwater in Wales, 1986-2015. Based on SMP data;
view the methods
of analysis (PDF 158 kb).

Productivity data are currently collected mostly
from colonies on Skomer (since 1991) and Bardsey (Gwynedd, since
1996), with the addition of data from Skokholm since 2013. Since
1995, mean productivity at colonies in Wales has been
relatively high but variable. Detailed monitoring and
reporting has been undertaken on Skomer and highlights some
problems faced by the shearwaters breeding there. Very low
productivity was recorded between 1992 and 1994; the
reasons for this are largely unknown although very wet weather
in May 1993 (the poorest breeding season to date) flooded many
burrows on Skomer. A lack of food may have affected breeding
success on Skomer in 2007 and 2008; annual comparison of chick
growth and adult food provisioning behaviour found that birds bred
later and chicks attained lower peak and fledging masses than in
any previous recorded year dating back to 19655. These changes were
accompanied by a reduction in parental attendance at the colony,
which was probably the result of parents switching to a dual
foraging strategy in 2007 and 2008. These events were linked to
higher sea surface temperature in the preceding winter and to a
reduction in prey quality, as indicated by the mean body mass of
two-year-old herring5.

On Skomer in 2012, lack of food around hatching time
was considered to have had the highest effect on productivity (0.55
fledged young per egg laid) although wet weather and the flooding
of a small number of burrows may also have contributed. In
2013, productivity increased slightly (0.60) and in 2014 the
highest productivity was recorded since 1998 (0.71). Breeding
success in 2015 was 0.60. This is slightly lower than the five year
average of 0.63 (2011-2015) and the 1995-2015 average of 0.62. It
is also 0.11 lower than the 2014 productivity of 0.716.
On Bardsey, flooding also reduced breeding success in some
parts of the study area in 2012, although this was compensated
for by high values of success in drier areas. A greater threat on
Bardsey are carrion crowsCorvus coronewhich
exploited accessible nest chambers in several years between
2002 and 2007 at least; the number of eggs removed ranged from 80
(in 2002) to 'several' (in 2007). However, in 2013, predation by corvids was not thought to be
significantly higher than in previous years. Instead, low success
of 0.61 chicks per breeding pair (2004-2012 average
0.73+0.02) was possibly
due to cold and generally poor weather throughout the spring. Low
productivity was consistent across the differing habitats of the
island; burrows in lowland banks and walls, mountain slopes and sea
cliff turf areas all performed poorly. None of the study
burrows contained abandoned eggs which would indicate failure at an
early stage7. In 2014
and 2015, breeding success on Bardsey was at 0.70 and 0.66 chicks
fledged per pair, respectively.

Population estimates and change 1969-2002 (census data)

Operation Seafarer

(1969-70)

Seabird Colony Register

(1985-88)

Seabird 2000

(1998-2002)

Population estimate (AOS*)

N/a

N/a

4,633

% change since previous
census

N/a

N/a

N/a

* AOS = Apparently Occupied Sites

Breeding Abundance

As for the other countries in the UK, the first
comprehensive estimates of population size of Manx shearwater in
Northern Ireland were obtained during the Seabird 2000
census. Only two colonies are known, both in the Copeland
Islands (Co. Down); Big Copeland was estimated to hold 1,766 AOS
with a further 2,867 AOS on nearby Lighthouse Island (total
4,633). Both islands have only been re-surveyed once since
Seabird 2000, in 2007, when 1,406 AOS were recorded on
Big Copeland and 3,444 AOS on Lighthouse Island (total 4,850)
indicating numbers had changed little overall8. Changes at the respective
islands between these two censuses (-20% on Big Island and +20% on
Lighthouse) may be associated with logistical difficulties in
surveying this nocturnal, burrow-nesting species.

Productivity

Breeding success data has been collected near annually on
Lighthouse Island, one of the Copeland Islands, since 20078. On average 0.73 chicks were
fledged per pair per year up to 2013 (no data were collected in
2014 and 2015).

Population estimates and change 1969-2002 (census data)

Operation Seafarer

(1969-70)

Seabird Colony Register

(1985-88)

Seabird 2000

(1998-2002)

Population estimate (AOS*)

N/a

N/a

32,545

% change since previous
census

N/a

N/a

N/a

* AOS = Apparently Occupied Sites

Breeding Abundance

During Seabird 2000, 32,545 pairs of Manx shearwater were
recorded in the Republic of Ireland although several small
colonies each probably holding a few hundred pairs were not
surveyed. The main concentration of colonies was in the south-west
on the offshore islands of County Kerry, several of which each
held between 2,000-10,000 pairs. In County Galway, Cruagh held
3,286 pairs with small numbers on a couple of other islands. A
few small colonies were recorded in counties Wexford and
Dublin. Due to the logistical difficulty in monitoring this
nocturnal and burrow-nesting species, no information exists as to
population trends since Seabird 2000.

Productivity

No systematic data on the productivity of Manx shearwaters in
the Republic of Ireland have been submitted to the SMP. Thus, data
is only available from Northern Ireland (see relevant section of
report).

Population estimates and change 1969-2002 (census data)

Operation Seafarer

(1969-70)

Seabird Colony Register

(1985-88)

Seabird 2000

(1998-2002)

Population estimate (AOS*)

N/a

N/a

37,178

% change since previous
census

N/a

N/a

N/a

* AOS = Apparently Occupied Sites

Breeding Abundance

The first comprehensive estimates of population size of Manx
shearwater for Ireland were obtained during the Seabird 2000
Census, when 37,178 pairs were estimated. The majority of
these (approximately 33,000 pairs) were found in the Republic of
Ireland where many colonies exist on offshore islands. In contrast,
only two colonies are known in Northern Ireland, both in the
Copeland Islands. During Seabird 2000, Big Copeland was
estimated to hold 1,766 pairs with a further 2,867 pairs on nearby
Lighthouse Island (total 4,633). A repeat census of these
islands in 2007 recorded 1,406 pairs on Big Copeland and 3,444
pairs on Lighthouse Island (total 4,850) indicating numbers had
changed little overall. Changes at the respective islands between
these two censuses (-20% on Big Island and +20% on Lighthouse) may
be associated with logistical difficulties in surveying this
nocturnal, burrow-nesting species. For the same reason, no other
information exists as to population trends for the whole of Ireland
since Seabird 2000.

Productivity

No systematic data on the productivity of Manx
shearwaters in the Republic of Ireland have been
submitted to the SMP. Thus, data is only available from Northern
Ireland where breeding success has been monitored near
annually on Lighthouse Island, one of the Copeland Islands, since
20078. On average 0.73 chicks were fledged per pair per
year up to 2013 (no data was collected in 2014 and 2015).

Population estimates and change 1969-2002 (census data)

Operation Seafarer

(1969-70)

Seabird Colony Register

(1985-88)

Seabird 2000

(1998-2002)

Population estimate (AOS*)

N/a

N/a

34

% change since previous
census

N/a

N/a

N/a

* AOS = Apparently Occupied Sites

Breeding Abundance

The population of Manx shearwaters on the Isle of Man is small
and confined to the Calf of Man; during Seabird 2000 only 34
AOS were recorded. In 2005, tape playback methods obtained
responses from 104 burrows and 91 burrows were found to be
occupied in 2010. With the completion of a rat eradication
programme on the Calf of Man in recent years the population has
increased substantially; counts of apparently occupied burrows
between 2011 and 2014 recorded 127, 147, 305 and 424 in each year
respectively. No data have been provided to the SMP for 2015.

Productivity

No systematic data on the productivity of Manx shearwaters on
the Isle of Man have been submitted to the SMP.

Population estimates and change 1969-2002 (census data)

Operation Seafarer

(1969-70)

Seabird Colony Register

(1985-88)

Seabird 2000

(1998-2002)

Population estimate (AOS*)

N/a

N/a

10

% change since previous
census

N/a

N/a

N/a

* AOS = Apparently Occupied Sites

Breeding Abundance

The known population of Manx shearwaters on the Channel Islands
is small at only 10 AOS. These were split evenly between
Jethou and Little Sark. There has been no assessment of the
population since Seabird 2000

Productivity

No systematic data on the productivity of Manx shearwaters on
the Channel Islands have been submitted to the SMP.

UK phenology, diet, survival rate

Phenology

No systematic data on phenology (timing of life-cycle events)
have been collected as part of the SMP.

Diet

No data on diet have been collected as part of the SMP.

Survival rate

Figure 1 shows the survival rate of adult Manx shearwaters at
Skomer, the only colony at which this parameter is monitored in the
UK. Although adult breeding survival varied greatly since 1992,
there is a clear decline since 2007 in breeding adult survival,
which may be a potential concern for Manx Shearwaters on Skomer
Island6.

The survival rate for adult breeding Manx
Shearwaters at Skomer from 2013 to 2014 was 0.84, slightly below
the study average (1978-2013: 0.86) As reported previously, these
survival estimates remain low, both in comparison with more
detailed studies carried out in the 1960s and 70s on Skokholm and
with what might be expected for a bird with such a low reproductive
rate. Survival has been particularly low recently for
unknown reasons.

References

2 Brown, A., Price, D., Slader,
P., Booker, H., Lock, L. and Deveney, D. 2011. Seabirds
on Lundy: their current status, recent history and prospects for
the restoration of a once important bird area. British
Birds 104: 139-158.

Partners

Data have been provided to the SMP by the generous contributions
of its partners, other organisations and volunteers throughout
Britain and Ireland. Partners to the SMP are: BirdWatch Ireland;
The British Trust for Ornithology; Centre for Ecology and
Hydrology; Natural Resources Wales; Department of Environment,
Food and Agriculture (Isle of Man); Department of Environment,
Heritage and Local Government (Republic of Ireland); States of
Guernsey Government; JNCC; Manx Birdlife; Manx National Heritage;
The National Trust; National Trust for Scotland; Natural England;
Northern Ireland Environment Agency; The Royal Society for the
Protection of Birds; Scottish Natural Heritage; Seabird Group;
Shetland Oil Terminal Environmental Advisory Group; Scottish
Wildlife Trust. More about the SMP partners >>